This invention relates generally to freight train brake control systems, and more particularly, to an apparatus and method for a pneumatically controlled graduated release of brake cylinder pressure in a freight train brake control system.
In conventional freight train braking systems controlled by a pneumatic control valve, pressurized fluid is utilized to control braking functions on each car of the train. One or more locomotives and each car are interconnected by a brake pipe which supplies pressurized fluid from a main reservoir on the locomotive to reservoirs on each car. Each car in a standard pneumatically operated freight braking system has onboard a reservoir, typically divided into emergency and auxiliary compartments which are charged via the brake pipe, a pneumatic control valve (PCV) and a fluid pressure activated brake cylinder device. The PCV selectively communicates between the brake pipe, each reservoir compartment, the brake cylinder device and the atmosphere. The PCV controls the operation of the brakes on the car by controlling the access of pressurized fluid between the brake cylinder and the reservoirs or the atmosphere. The operation of the PCV is controlled by the engineer from the locomotive by adjusting the pressure in the brake pipe. Basically, a reduction in brake pipe pressure signals the PCV to admit pressurized fluid from a reservoir into the brake cylinder device to apply the brakes. Conversely, an increase in brake pipe pressure signals the PCV to vent the brake cylinder to the atmosphere thereby releasing the brakes.
Overall, the conventional pneumatic braking system utilizing the PCV has proven to be a very safe and reliable system. However, a couple of disadvantages of the PCV controlled system are that it takes time for the pressure change initiated at the locomotive to propagate throughout what may be hundreds of cars. As a result, the PCV on each car senses the pressure change sequentially such that the brakes on each car are applied in sequence rather than simultaneously. The PCV on cars near the locomotive will sense the pressure change sooner and thus the brakes on that car will be applied in advance of the brakes on cars down the line.
Another disadvantage is that once the brakes are applied, the only way to release them is to vent the brake cylinder to atmosphere, thus completely exhausting all pressure from the brake cylinder. In other words, the brake cylinder pressure cannot be partially reduced. Either all of the pressure must be kept or it all must be dumped. Furthermore, once the brake cylinder pressure is vented it can take time to recharge the brake pipe and reservoirs sufficiently to make further brake applications.
Prior art devices include the use of retainer valves with ABDW, ABDX type equipment to “retain” a portion of the brake cylinder pressure upon a direct release.
A relatively recent development in freight train braking controls is the Electrically Controlled Pneumatic (ECP) brake control system. In a conventional ECP system, an electronic controller (EC) is provided on each car along with solenoid operated valves which control the exchange of pressure between the brake cylinder device and the reservoirs or the atmosphere, basically taking over the functions of the PCV. Thus, the EC directly controls the brake cylinder and may be referred to as a brake cylinder control (BCC) type ECP system.
Initially, this BCC type ECP system has been tested as an overlay system on the conventional pneumatic system, with the PCV functioning as a back-up brake control device. However, all electronic BCC type ECP brake control systems are being prepared and the American Association of Railroads (AAR) is in the process of promulgating certain requirements regarding minimum equipment and operating conditions for such ECP systems.
One of the advantages of the BCC type system is that the EC is electrically signaled from the locomotive to operate the brake cylinder device. Thus, the brake signal is propagated essentially instantaneously and the brakes on every car can be actuated at virtually the same time. Another advantage is that the level of brake cylinder pressure is adjustable because the solenoid valves can partially vent brake cylinder pressure without completely exhausting all of the pressure to the atmosphere. As a result, the engineer can signal the EC to increase or decrease the braking force by any amount desired.
However, one disadvantage is the cost of implementing such an ECP system. For example, the AAR minimum requirements include, among other things, the requirement of a 2500 W power source on the locomotive, a 230 VDC trainline cable and a communications device one every car each with having a battery as a back-up power source. These requirements impose a significant cost factor.
Accordingly, there is a need for a device which provides a pneumatically controllable graduated release of brake cylinder pressure to obtain the graduated release advantages of the ECP system without the need for all of the associated electrical equipment and costs.